Oil and gas reservoirs may be exploited by tapping the resources therein via wellbores. Drilling of wellbores may require drilling a considerable distance into the earth. Many oil & gas bearing formations are at sub-sea locations. The direction of drilling may vary from a vertical position to a horizontal position. The wellbore created by drilling may be stabilised by use of casing or lining or by other measures. Strings assembled from stands of tubular drill pipe with a formation material removal assembly at the lower end of the string are used to form boreholes by drilling, and to conduct other wellbore operations using tools and instruments deployed upon or incorporated in the drill string (otherwise known as a work string if no drilling bottom hole assembly is attached). Use of downhole telemetry (RFID, WIFI etc.) to locate and position instruments and tools is desirable.